Standardization efforts for the wireless LAN standard in IEEE 802.11 so far have mainly targeted indoor communication and has successively added physical layer standards mainly focusing on an increase in transmission capacity such as 802.11b (maximum 11 Mbps), 802.11a, 11g (maximum 54 Mbps), 802.11n (maximum 600 Mbps), and 802.11ac (maximum 6.9 Gbps). Meanwhile, as examination of smart meters to implement a smart grid has been well under way, the need for low rate and long-distance outdoor transmission is also increasing. There are also ongoing discussions about assignment of available specified low power radio frequencies and the like intended for such applications. Against this background, studies aiming at development of new communication standards using a sub GHz band (frequency band slightly lower than 1 GHz) have started and IEEE802.11 started up TGah (802.11ah), a task group working on wireless LAN standards using sub GHz frequency bands in 2010. A principal required specification in TGah (802.11ah) is “data rate of 100 kbps or higher and maximum transmission distance of 1 km.”
IEEE 802.11a and subsequent standards including TGah (802.11ah) using an OFDM modulation scheme establish various kinds of synchronization using a preamble at the beginning of a packet to perform burst communication. A preamble is constructed of an STF (Short Training Field, which may also be called “short preamble”) used for AGC (Automatic Gain Control) or coarse adjustment AFC (Automatic Frequency Control) and LTF (Long Training Field, which may also be called “long preamble”) used for estimation of fine adjustment AFC or transmission path characteristic. In the STF, ten STS (Short Training Symbol) symbols are repeatedly mapped in the time domain and in the LTF, two LTS (Long Training Symbol) symbols are repeatedly mapped in the time domain.
In digital transmission such as OFDM, a carrier frequency generated on the receiving side normally does not match a carrier frequency on the transmitting side, and therefore, the receiving side needs to detect an error (carrier frequency error) in the carrier frequency and perform AFC for correcting the carrier frequency. The carrier frequency error appears as phase rotation of constellation of a received signal for each sample.
For example, NPL 1 discloses a method for performing AFC using a preamble made up of an STF and an LTF as a scheme for detecting/correcting carrier frequency errors in 802.11a. More specifically, this method calculates an auto-correlation between a received signal and a signal obtained by causing the received signal to delay by a repetition period (time corresponding to 1 symbol) using repetition mapping of an STF or LTF, calculates an angle component (phase component) of the signal obtained, and thereby detects a carrier frequency error.
FIG. 9 illustrates a configuration of AFC section 10 to which the method disclosed in NPL 1 is applied.
Correlation calculation section 1 calculates a correlation using repetition mapping of STS. More specifically, correlation calculation section 1 calculates a correlation between an STS at a certain time in the STF and an STS delayed by a repetition period (time corresponding to 1 STS). Detection section 2 detects a phase component obtained from the correlation result of correlation calculation section 1 as a phase rotation amount after a lapse of time corresponding to the repetition period, that is, a carrier frequency error. Correction section 3 corrects the carrier frequency using the carrier frequency error obtained in detection section 2.
Here, a general carrier frequency error detection method (processing by correlation calculation section 1 and detection section 2) will be described. Here, of STSs repeatedly mapped, an n-th STS is expressed by equation 1 or 2. In equation 1, An denotes amplitude of an STS.[1]STSn=Anexp(j2πft) n=0,1,2,  (Equation 1)[2]an(t)=STSn  (Equation 2)
In this case, an STS delayed by repetition period S (time corresponding to 1 STS) with respect to the n-th STS (STSn) corresponds to an (n+1)th STS (STSn+1) as shown in equation 3.[3]an(t+S)=STSn+1  (Equation 3)
Here, for example, when a carrier frequency error resulting from a phase rotating by a phase amount δ [degree] every lapse of a time of period S is included in a received signal, STSn shown in equation 1 is expressed by following equation 4.
                    (                  Equation          ⁢                                          ⁢          4                )                                                                                  STS            n                    =                                    A              n                        ⁢                                                  ⁢                          exp              ⁡                              (                                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    f                    ⁢                                                                                  ⁢                    t                                    +                                      2                    ⁢                    π                    ⁢                                                                  n                        ⁢                                                                                                  ⁢                        δ                                            360                                                                      )                                                    ⁢                                  ⁢                              n            =            0                    ,          1          ,          2          ,          …          ⁢                                          ,          9                                    [        4        ]            
Thus, the result of calculation of an auto-correlation between the n-th STS (STSn) and the (n+1)th STS (STSn+1) (complex multiplication of the STSn by a complex conjugate of the STSn+1 or complex multiplication of the STSn+1 by a complex conjugate of the STSn) is expressed by equation 5.
                                              ⁢                  (                      Equation            ⁢                                                  ⁢            5                    )                                                                                          STS                          n              +              1                                *                      STS            n            *                          =                                            A                              n                +                1                                      ⁢                          exp              ⁡                              (                                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    f                    ⁢                                                                                  ⁢                    t                                    +                                      2                    ⁢                    π                    ⁢                                                                                            (                                                      n                            +                            1                                                    )                                                ⁢                        δ                                            360                                                                      )                                      *                          A              n                        ⁢                          exp              ⁡                              (                                                                            -                      j                                        ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    f                    ⁢                                                                                  ⁢                    t                                    -                                      2                    ⁢                    π                    ⁢                                                                                            n                          )                                                ⁢                        δ                                            360                                                                      )                                              =                                    A                              n                +                1                                      ⁢                          A              n                        ⁢                          exp              ⁡                              (                                  2                  ⁢                  π                  ⁢                                      δ                    360                                                  )                                                                        [        5        ]            
By extracting the phase component (angle component) in the correlation calculation result shown in equation 5, a phase rotation amount δ after a lapse of time, that is, a carrier frequency error is detected.
A general method for detecting a carrier frequency error has been described so far.
PTL 1 discloses a method for detecting a carrier frequency error between the transmitting side and the receiving side using an initial packet at a start of communication and correcting the carrier frequency using the carrier frequency error at the time of receiving a subsequent packet.